Cross-linked polymer electrolyte membranes

ABSTRACT

Crosslinked polymers are produced by polymerizing a styrene-based comonomer with a bifunctional styrenated crosslinkable monomer comprising the following straight chain formula: CH 2 ═CH—C 6 H 4 —CH 2 —(OCH 2 CH 2 ) n —O—CH 2 —C 6 H 4 —CH═CH 2 . The styrenated crosslinkable monomer can be produced from a two arm polyethylene glycol having a molecular weight between 200 g/mol and 35,000 g/mol. The styrenated crosslinkable monomer can also be produced from a two arm polyethylene oxide having a molecular weight between 100 kg/mol and 800 kg/mol. The styrenated crosslinkable monomer can also be produced from a four arm polyethylene glycol. Polymer electrolyte membranes are produced from the crosslinked polymers.

TECHNICAL FIELD

This disclosure relates to novel cross-linked styrene-based electrolytemembranes, and in particular to cross-linked polymer electrolytemembranes produced by crosslinking bifunctional monomers based onpolyethylene glycols or polyethylene oxides with a sulfonic acid bearingco-monomer.

BACKGROUND

PEM fuel cells (PEMFCs) generate power from electrochemical conversionof fuels such as hydrogen and hydrocarbons at its anode and oxidantssuch as oxygen and air at its cathode using a membrane as electrolyte.The membrane acts both as a proton conductor and a barrier between thefuel and oxidants. Developing a membrane with high ionic conductivity athigh temperature and low relative humidity (RH %) is desired to simplifythe humidification system and operation, improve fuel cell performance,and reduce the cost for early commercialization of fuel cell electricvehicles. Current state-of-the-art membranes such as Nafion™ membranesand other perfluorosulfonic acid (PFSA) membranes have reasonableconductivity at high RH % and at temperatures below 100° C. However,these membranes hold less water at low RH % and undergo permanentthermal degradation at temperatures above 100° C.

In these membranes, conductivity at low RH % could be improved byincreasing the acid content (—SO₃H group) or by reducing the equivalentweight (EW). However, increasing the acid content beyond certain valuesleads to polymer dissolution, weak mechanical structure, and eventuallyfailure of the membrane in fuel cells. The linear-chain-structure incurrent state-of-the-art PFSA membranes is inadequate to allow acidcontent beyond certain values. Without increasing the acid content andpreventing polymer structure damage at high temperature, currentstate-of-the-art PFSA membranes are unable to function at low RH % andat high temperature. In addition, these current PFSA membranes aremanufactured under extremely high reaction conditions usingsophisticated equipment and processes that make them difficult andexpensive to produce.

SUMMARY

Disclosed herein are crosslinked polymers produced by polymerizing astyrene-based comonomer with a bifunctional styrenated crosslinkablemonomer comprising the following straight chain formula:CH₂═CH—C₆H₄—CH₂—(OCH₂CH₂)_(n)—O—CH₂—C₆H₄—CH═CH₂. The styrenatedcrosslinkable monomer can be produced from a two arm polyethylene glycolhaving a molecular weight between 200 g/mol and 35,000 g/mol. Thestyrenated crosslinkable monomer can also be produced from a two armpolyethylene oxide having a molecular weight between 100 kg/mol and 800kg/mol. The styrenated crosslinkable monomer can also be produced from afour arm polyethylene glycol.

Also disclosed herein are polymer electrolyte membranes comprised of thecrosslinked polymers. Crosslinking bifunctional styrentated monomerswith a sulfonic acid bearing comonomer produces membranes with very highacid content and strong polymer structure.

These and other aspects of the present disclosure are disclosed in thefollowing detailed description of the embodiments, the appended claimsand the accompanying FIGURES.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features, advantages and other uses of the present apparatuswill become more apparent by referring to the following detaileddescription and drawing in which:

FIG. 1 is a schematic of a membrane electrode assembly of a fuel cellincorporating a crosslinked polymer as disclosed herein.

DETAILED DESCRIPTION

The inventors' objective of developing PFSA membranes capable offunctioning at low RH % and at high temperature is realized by theirdevelopment of styrenated crosslinkable monomers and the crosslinkedpolymers produced from the inventive monomers. The bifunctionalstyrene-based liquid monomers disclosed herein have a very strong middlesegment that can be readily produced without the need for extremely highreaction conditions and the sophisticated equipment and processes thatrenders production difficult and expensive. The styrenated crosslinkablemonomers can easily be polymerized with many types of comonomer todevelop varieties of membranes. The crosslinked polymers disclosedherein provide membranes with very low equivalent weight that can retainthe morphological structure at high temperatures while maintainingconductivity at low RH %.

The inventors have discovered that when crosslinkers, such as somecommercially available crosslinkers, with fragile or weak middlesegments are used to develop membranes, these membranes are notmechanically strong. In addition, these crosslinkers do not allow forthe increase in acid content in the membrane. In some instances, acidbearing side groups of membranes are also ionically crosslinked. Butthis type of crosslinking is not stable and unravels under liquid waterand high temperature.

The styrenated crosslinkable monomers disclosed herein have a strongmiddle segment to prevent dissolution of the membrane, resulting in acrosslinked polymer having a fluorinated segment between benzene and—SO₃H that is highly acidic. No weak middle segments are incorporatedbetween the two end reactive groups to fabricate the membrane. Themembranes disclosed herein, produced with the styrenated crosslinkablemonomers and a compatible comonomer, incorporate acid functionality tothe membrane. The disclosed styrenated crosslinkable monomers can beused to develop other types of polymer materials as well.

The styrenated crosslinkable monomers disclosed herein have thefollowing straight chain formula:CH₂═CH—C₆H₄—CH₂—(OCH₂CH₂)_(n)—O—CH₂—C₆H₄—CH═CH₂with the middle segment having either a molecular weight between 200g/mol and 35,000 g/mol or between 100,000 g/mol and 800,000 g/mol.Depending on the molecular weight, the styrenated crosslinkable monomeris referred to as a styrenated polyethylene glycol (sPEG) or astyrenated polyethylene oxide (sPEO).

The styrenated crosslinkable monomer can be produced from a two armpolyethylene glycol (PEG) of the formula H—(OCH₂CH₂)n-OH having amolecular weight between 200 g/mol and 35,000 g/mol.

The styrenated crosslinkable monomer can also be produced from a two armpolyethylene oxide (PEO) of the formula H—(OCH₂CH₂)n-OH having amolecular weight between 100,000 g/mol and 800,000 g/mol.

The styrenated crosslinkable monomer can also be produced from a fourarm PEG of the following formula:

The styrenated crosslinkable monomer is produced by functionalizing eacharm of the PEG or PEO with styrene. Functionalizing can be done bymixing the PEG or PEO with vinyl benzyl chloride (CH₂═CH—C₆H₄—CH₂—Cl),for example. The PEG or PEO and vinyl benzyl chloride can be mixed in asolvent and reacted using a base such as potassium hydroxide tofunctionalize the arms with styrene at each end, with the reactioncarried out at room temperature until completion. The product isquenched with an acid and filtered. The filtered product is dried andthe styrenated crosslinkable monomer is precipitated using diethylether. The styrenated crosslinkable monomer is highly reactive,warranting storage at very low temperature.

Crosslinked polymers can be produced from the styrenated crosslinkablemonomer of formula CH₂═CH—C₆H₄—CH₂—(OCH₂CH₂)_(n)—O—CH₂—C₆H₄—CH═CH₂,thereby producing membranes with very high acid content that can retainthe morphological structure at high temperatures while maintainingconductivity at low RH %. Such crosslinked polymers are produced bypolymerizing a styrene-based comonomer with the styrenated crosslinkablemonomer sPEG or sPEO. The crosslinked polymer will have the followingStructure 1, with n being dependent on the molecular weight of thestyrenated crosslinkable monomer used:

In the crosslinked polymer formula above, X is SO₃H⁺ or R_(f)—SO₃H⁺,depending on the styrene-based comonomer used and the polymerizationprocess used. If X=R_(f)—SO₃H⁺, then R_(f)=—(CF₂)_(m)—O—CF₂CF₂— andm=2-7. Specific examples of crosslinked polymers will be describedherein under Examples.

To develop a pure fluorinated membrane, the crosslinked polymer ofStructure 1 can be fluorinated with elemental fluorine gas to converthydrogen elements into fluorine, resulting in a fluorinated crosslinkedpolymer having the following Structure 2:

The crosslinked polymers in Structure 1 and the fluorinated crosslinkedpolymers in Structure 2 can incorporate free-radical scavengers toimprove the chemical durability of the membranes during, for example,open circuit voltage hold conditions. Free-radical scavengers such as4-hydroxy styrene or 4-vinylaniline are co-added with the styrene-basedcomonomer prior to polymerization to neutralize free-radicals OH., OOH.responsible for chemical degradation of membrane. Although cerium oxideor other inorganic additives have been added in polymer electrolytemembranes to improve chemical durability, these additives can leach outfrom the membrane, compromising the durability. It is believed that thepolymer based additives (4-hydroxy styrene or 4-vinyl aniline) used asfree-radicals scavengers to improve the durability are more stable thanthe inorganic fillers in the membrane. The crosslinked polymer inStructure 1 polymerized with one of 4-hydroxy styrene or 4-vinylanilineas a free-radical scavenger will result in the crosslinked polymer shownin Structure 3, where Z═OH or NH₂:

The crosslinked polymers shown in any of Structure 1, 2 or 3 can be usedto produce a polymer electrolyte membrane for use in a fuel cell, forexample. To fabricate a composite membrane, the crosslinked polymer isimbibed into a porous support, including ePTFE, a nanofiber support orany other support, polymerized, hydrolyzed, and ion-exchanged.

The styrenated crosslinkable monomer sPEG or sPEO disclosed herein canalso be used to produce crosslinked membranes for alkaline fuel cellsand direct methanol fuel cells. For alkaline fuel cells, a comonomersuch as vinyl benzyl trimethyl chloride/hydroxide can be polymerizedwith styrenated crosslinkable monomer sPEG or sPEO to produce an ionexchange membrane with very low equivalent weight, high conductivity,and a strong structure. These styrenated crosslinkable monomers can alsobe used with other types of comonomers to develop an ion exchangemembrane. For direct methanol fuel cells, crosslinked membranes can bedeveloped with low equivalent weight and strong structure because directmethanol fuel cells also use proton exchange membranes similar tohydrogen fuel cells. Since styrenated crosslinkable monomer sPEG or sPEOare styrene-based bifunctional monomers and styrene has very highreactivity and is easily polymerizable, they can also be copolymerizedwith many types of compatible non-styrene based comonomers to developcrosslinked polymers.

FIG. 1 illustrates the use of a membrane produced with a crosslinkedpolymer disclosed herein. FIG. 1 is a schematic of a fuel cell 70, aplurality of which makes a fuel cell stack. The fuel cell 70 iscomprised of a single membrane electrode assembly 20. The membraneelectrode assembly 20 has a membrane 80 made from a crosslinked polymerdisclosed herein, the membrane 80 coated with catalyst 84 with a gasdiffusion layer 82 on opposing sides of the membrane 80. The membrane 80has a catalyst layer 84 formed on opposing surfaces of the membrane 80,such that when assembled, the catalyst layers are each between themembrane 80 and a gas diffusion layer 82. Alternatively, a gas diffusionelectrode is made by forming one catalyst layer 84 on a surface of twogas diffusion layers 82 and sandwiching the membrane 80 between the gasdiffusion layers 82 such that the catalyst layers 84 contact themembrane 80. When fuel, such as hydrogen gas (shown as H₂), isintroduced into the fuel cell 70, the catalyst layer 84 of the catalystcoated membrane 80 splits hydrogen gas molecules into protons andelectrons. The protons pass through the membrane 80 to react with theoxidant (shown as O₂), such as oxygen or air, forming water (H₂O). Theelectrons (e⁻), which cannot pass through the membrane 80, must travelaround it, thus creating the source of electrical energy.

Examples of crosslinked polymers produced by one of the styrenatedcrosslinkable monomers disclosed herein and for use in polymerelectrolyte membranes are described in greater detail. Each of thecrosslinked polymers described below can be fluorinated as shown inStructure 2, and/or can be produced with free-radical scavengers, asshown in Structure 3.

A crosslinked polymer is produced by mixing the desired ratio ofstyrenated crosslinkable monomer sPEG or sPEO with styrene sulfonic acidcomonomer having the formula CH₂═CH—C₆H₄—SO₃H⁺, along with afree-radical initiator such as azobisisobutyronitrile (AIBN) or benzoylperoxide to initiate the polymerization reaction, the mixturepolymerized under heat or UV light. The resulting crosslinked polymerhas the following Structure 4:

The crosslinked polymer shown in Structure 4 is also produced by mixingthe desired ratio of styrenated crosslinkable monomer sPEG or sPEO withstyrene sulfonate-sodium comonomer having the formulaCH₂═CH—C₆H₄—SO₃Na⁺, the mixture polymerized under heat or UV light. Theintermediate polymer structure shown below

further undergoes ion exchange with an acid solution to produce thecrosslinked polymer shown in Structure 4.

Another crosslinked polymer is produced by mixing the desired ratio ofstyrenated crosslinkable monomer sPEG or sPEO with styrene sulfonylhalide comonomer having the formula CH₂═CH—C₆H₄—SO₂Cl orCH₂═CH—C₆H₄—SO₂F, the mixture polymerized under heat or UV light. Theintermediate polymer structure shown below

is hydrolyzed with a base/alcohol solution and undergoes ion exchangewith an acid solution to produce the crosslinked polymer shown inStructure 4.

To produce the crosslinked polymer shown in Structure 4 withfree-radical scavengers, CH₂═CH—C₆H₄—OH or CH₂═CH—C₆H₄—NH₂ is added at1-2 wt % to the mixture during polymerization, resulting in acrosslinked polymer having free-radical scavengers, shown in Structure 5below:

Another crosslinked polymer is produced by mixing the desired ratio ofstyrenated crosslinkable monomer sPEG or sPEO with 4-Bromo styrenecomonomer having the formula CH₂═CH—C₆H₄—Br, the mixture polymerizedunder heat or UV light. The intermediate polymer structure shown below

is reacted with I—(CF₂)_(m)—O—CF₂—CF₂—SO₂F, where m=2-7, under heat andin the presence of copper or copper oxide catalyst to produce a secondintermediate polymer structure shown below in Structure 6:

The second intermediate polymer structure is hydrolyzed with abase/alcohol solution and undergoes ion exchange with an acid solutionto produce the crosslinked polymer shown in Structure 7 below, whereR_(f)=—(CF₂)_(m)—O—CF₂CF₂— and m=2-7.

Another crosslinked polymer is produced by mixing the desired ratio ofstyrenated crosslinkable monomer sPEG or sPEO with 4-hydroxy styrenecomonomer having the formula CH₂═CH—C₆H₄—OH, the mixture polymerizedunder heat or UV light. The intermediate polymer structure shown below

is reacted with I—(CF₂)_(m)—O—CF₂—CF₂—SO₂F, where m=2-7, under heat andin the presence of copper or copper oxide catalyst to produce a secondintermediate polymer structure shown in Structure 8.

The second intermediate polymer structure is hydrolyzed with abase/alcohol solution and undergoes ion exchange with an acid solutionto produce the crosslinked polymer shown in Structure 7, where, again,R_(f)=—(CF₂)_(m)—O—CF₂CF₂— and m=2-7.

To produce the crosslinked polymer structure shown in Structures 7 or 8with free-radical scavengers, 4-vinyl aniline (CH₂═CH—C₆H₄—NH₂) is addedat 1-2 wt % to the mixture during polymerization, resulting in acrosslinked membrane structure having free-radical scavengers. Structure9, below, illustrates the crosslinked polymer of Structure 7 withfree-radical scavengers:

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiments but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures as is permitted under the law.

What is claimed is:
 1. A polymer electrolyte membrane for a fuel cellcomprising a crosslinked polymer produced by polymerizing astyrene-based comonomer with a styrenated crosslinkable monomercomprising the following straight chain formula:CH₂═CH—C₆H₄—CH₂—(OCH₂CH₂)_(n)—O—CH₂—C₆H₄—CH═CH₂, wherein the crosslinkedpolymer has the following Formula A:

wherein X=SO₃H⁺ or R_(f)—SO₃H⁺, with R_(f)=—(CF₂)_(m)—O—CF₂CF₂— andm=2-7 in which R_(f) is obtained from I—(CF₂)_(m)—O—CF₂—CF₂—SO₂F, withm=2-7.
 2. The polymer electrolyte membrane of claim 1, wherein thestyrenated crosslinkable monomer is produced from a two arm polyethyleneglycol having a molecular weight between 200 g/mol and 35,000 g/mol. 3.The polymer electrolyte membrane of claim 1, wherein the styrenatedcrosslinkable monomer is produced from a two arm polyethylene oxidehaving a molecular weight between 100 kg/mol and 800 kg/mol.
 4. Thepolymer electrolyte membrane of claim 1, wherein the styrenatedcrosslinkable monomer is produced from a four arm polyethylene glycol.5. The polymer electrolyte membrane of claim 1, wherein the crosslinkedpolymer is a fluorinated crosslinked polymer having the followingformula:


6. The polymer electrolyte membrane of claim 1, wherein the crosslinkedpolymer is further polymerized with a free radical scavenger.
 7. Thepolymer electrolyte membrane of claim 1, wherein the crosslinked polymeris further polymerized with a free radical scavenger selected from4-hydroxy styrene and 4-vinylaniline, the crosslinked polymer havingfree radical scavengers and shown in the following formula, wherein Z=OHor NH₂:


8. The polymer electrolyte membrane of claim 1, wherein thestyrene-based comonomer has the formula CH₂═CH—C₆H₄—SO₃H⁺ and thecrosslinked polymer has the Formula A wherein X=SO₃H⁺.
 9. The polymerelectrolyte membrane of claim 1, wherein the styrene-based comonomer hasthe formula CH₂═CH—C₆H₄—SO₃Na⁺ and the crosslinked polymer has theFormula A wherein X=SO₃H⁺.
 10. The polymer electrolyte membrane of claim1, wherein the styrene-based comonomer has the formula CH₂═CH—C₆H₄—SO₂Clor CH₂═CH—C₆H₄—SO₂F and the crosslinked polymer has the Formula Awherein X=SO₃H⁺.
 11. The polymer electrolyte membrane of claim 1,wherein the styrene-based comonomer has the formula CH₂═CH—C₆H₄—Br, andthe crosslinked polymer has the Formula A wherein X=R_(f)—SO₃H⁺, withR_(f)=—(CF₂)_(m)—O—CF₂CF₂— and m=2-7 in which R_(f) is obtained fromI—(CF₂)_(m)—O—CF₂—CF₂—SO₂F, with m=2-7.
 12. The polymer electrolytemembrane of claim 1, wherein the styrene-based comonomer has the formulaCH₂═CH—C₆H₄—OH, and the crosslinked polymer has the Formula A whereinX=—O—R_(f)—SO₃H⁺, with R_(f)=—(CF₂)_(m)—O—CF₂CF₂— and m=2-7 in whichR_(f) is obtained from I—(CF₂)_(m)—O—CF₂—CF₂—SO₂F, m=2-7.
 13. Thepolymer electrolyte membrane of claim 1 further comprising a poroussupport on which the crosslinked polymer is supported.
 14. A polymerelectrolyte membrane for a fuel cell comprising a crosslinked polymerproduced by polymerizing a styrene-based comonomer with a styrenatedcrosslinkable monomer comprising the following straight chain formula:CH₂═CH—C₆H₄—CH₂—(OCH₂CH₂)_(n)—O—CH₂—C₆H₄—CH═CH₂, wherein the styrenatedcrosslinkable monomer is produced by functionalizing polyethyleneglycols with vinyl benzyl chloride, the crosslinked polymer having theformula:

wherein X=SO₃H⁺ or R_(f)—SO₃H⁺, with R_(f)=—(CF₂)_(m)—O—CF₂CF₂— andm=2-7 in which R_(f) is obtained from I—(CF₂)_(m)—O—CF₂—CF₂—SO₂F, m=2-7.15. The polymer electrolyte membrane of claim 14 further comprising aporous support on which the crosslinked polymer is supported.
 16. Thepolymer electrolyte membrane of claim 14, wherein the polyethyleneglycol has a molecular weight between 200 g/mol and 35,000 g/mol. 17.The polymer electrolyte membrane of claim 14, wherein the polyethyleneglycol is a two arm polyethylene oxide having a molecular weight between100 kg/mol and 800 kg/mol.